The present invention relates to a process for removing complexed or chelated cations from an aqueous solution using a cationic ion exchange resin. In particular, the invention relates to a process for removing heavy metal cations from a chelate or complex with cyanide and, more particularly, cadmium from a cadmium cyanide complex dissolved in the aqueous effluent of a plating bath such that the cadmium can be recycled or recovered for reuse and the cyanide is recycled or converted to a form which can be disposed of with minimal effect on the environment.
Electroplating baths are used to plate numerous different metals, such as cadmium, copper, zinc, silver, gold and nickel, on to a conductive substrate. One of the major problems with the use of electroplating baths is the need to dispose of high volumes of metals complexed or chelated with cyanides (referred to below as the cyanide complex) which are generated in the process and are present in the waste streams. The most common method of treating this waste stream is to react the cyanide complex with chlorine in the form of chlorine gas, sodium hypochlorite or calcium hypochlorite followed by the addition of an alkali, such as caustic soda, to raise the pH to 9 to 11 to convert the cyanide to cyanate and then to CO.sub.2 and N.sub.2, releasing the metal ion from the complex in the process. An alternative method is the use of ozone and hydrogen peroxide. The solution is then treated with hydroxide to form an insoluble precipitate of the heavy metal, the resulting metal hydroxide sludge being disposed of as hazardous waste as it is not in a form which can be readily recovered, recycled or reused.
An alternative recovery process involves the use of an ion exchange resin to remove certain ionic materials. Ion exchange resins are polymeric materials which have charged functional groups exposed to the aqueous feed stream. The charge on the functional group determines the type of ions which can be attracted by the resin. For example, some cationic resins typically contain sulfonic acid groups which are negatively charged and thus attract positively charged cations. Some anionic resins contain amine-based functional groups which are positively charged and thus attract anionic groups. Thus, anionic ion exchange resin can be used in place of alkaline chlorination to remove free cyanide as well as cyanide complexes such as Cd(CN).sub.4.sup.-2, Zn(CN).sub.4.sup.-2, Fe(CN).sub.4.sup.-2, or Fe(CN).sub.6.sup.-4. However, these complexes are strongly bound to the anionic resin and not readily removed by standard NaOH resin regeneration treatment, resulting in rapid degeneration of the resin and a short useful processing life.
Cationic resins can be used to remove cations from solution after the complex is broken with subsequent release by acid treatment. As an example, the Rohm and Haas Company markets Amberlite.RTM. IRC-178 chelating resin for the removal of cations from aqueous solutions. To obtain high selectivity the most effective operative conditions require the use of a highly acidic solution (pH=2.0), the selectivity rapidly decreasing as the pH is raised to neutral or basic. Other published material (U.S. Pat. Nos. 5,262,018 and 5,200,473) disclose that iminodiacetic acid resins produced by numerous other manufacturers, N-hydroxypropyloicolylamine functionalized chloromethylated polymer (Dowex.RTM. XFS-415 and XFS-43084) and chelating resins with amino phosphoric acid functional groups (Duolite.RTM. C-467) behave similarly for removing free ions of transition metals over the pH range of 0.5 to 4.0.
While strong acid cationic resins in the hydrogen form can be used to remove complexed heavy metals, strong acid cationic resins in the sodium form will not remove complexed heavy metals. However, when used on cyanide complexing solutions, the hydrogen form of cationic resins cause the cyanide complex to break down and release hydrogen cyanide gas which is highly toxic.
U.S. Pat. No. 5,200,473 to Jeanneret-Gris suggests that a chelating resin may be used to draw a metal ion out of a weak complex. However, complexes of metals with cyanide are extremely stable and known chelating resins can be unsuitable and undesirable when they produce toxic gases. In fact, such resins can be regenerated by washing the resin with a basic pH solution of cyanide ions, reversing the attraction of the chelating resin for the metal, i.e., forming instead of breaking the cyanide/metal ion complex.
U.S. Pat. No. 5,198,021 discloses the use of a guanidine based resin specifically designed to recover gold or silver complexed with cyanide. However, this resin, which is a hydrogen form resin, suffers from at least two deficiencies--the exchange results in the formation of highly toxic HCN gas and regeneration of the resin, i.e., removal of the absorbed metal, is performed using NaOH which would produce heavy metal hydroxides from which the free metal is not readily recovered.
Thus there is a need for a safe, economical, and reliable process which will allow the recovery of high volumes of heavy metals from wastewater streams in a form suitable for recovery or recycling while at the same time avoiding the production of hazardous waste and toxic gases.